Calcium-induced Calcium Release in Smooth Muscle ✪ Loose Coupling between the Action Potential and Calcium Release

Calcium-induced calcium release (CICR) has been observed in cardiac myocytes as elementary calcium release events (calcium sparks) associated with the opening of L-type Ca 2 1 channels. In heart cells, a tight coupling between the gating of single L-type Ca 2 1 channels and ryanodine receptors (RYRs) underlies calcium release. Here we demonstrate that L-type Ca 2 1 channels activate RYRs to produce CICR in smooth muscle cells in the form of Ca 2 1 sparks and propagated Ca 2 1 waves. However, unlike CICR in cardiac muscle, RYR channel opening is not tightly linked to the gating of L-type Ca 2 1 channels. L-type Ca 2 1 channels can open without triggering Ca 2 1 sparks and triggered Ca 2 1 sparks are often observed after channel closure. CICR is a function of the net flux of Ca 2 1 ions into the cytosol, rather than the single channel amplitude of L-type Ca 2 1 channels. Moreover, unlike CICR in striated muscle, calcium release is completely eliminated by cytosolic calcium buffering. Thus, L-type Ca 2 1 channels are loosely coupled to RYR through an increase in global [Ca 2 1 ] due to an increase in the effective distance between L-type Ca 2 1 channels and RYR, resulting in an uncoupling of the obligate relationship that exists in striated muscle between the action potential and calcium release. key words: calcium-induced calcium release • smooth muscle • Ca 2 1 sparks • excitation–contraction coupling • action potential signaling I N T R O D U C T I O N In striated muscle excitation–contraction (E-C) 1 coupling is initiated by the gating of sarcolemmal L-type Ca 2 1 channels, which trigger the release of calcium from ryanodine receptors (RYRs) on the sarcoplasmic reticulum (Endo, 1977; Fabiato, 1983; Nabauer et al., 1989; Tanabe et al., 1990; McPherson and Campbell, 1993). While the mechanism of coupling between L-type Ca 2 1 channels and RYRs is different in skeletal and cardiac myocytes, in both cell types local interactions between these proteins underlie calcium release. In skeletal myocytes, calcium entry is not required for calcium release (Armstrong et al., 1972), but gating of the L-type Ca 2 1 channel appears to be physically coupled to RYR opening (Tanabe et al., 1990; Nakai et al., 1998). Calcium entry through L-type Ca 2 1 channels triggers calcium-induced calcium release (CICR) in heart cells (Fabiato, 1985; Nabauer et al., 1989), resulting in localized calcium release, termed Ca 2 1 sparks (Cheng et al., 1993; Cannell et al., 1995; Lopez-Lopez et al., 1995). This coupling process involves a local increase in [Ca 2 1 ] i in the microdomain of the L-type Ca 2 1 channel, which is sensed by the RYR, resulting in RYR gating, and several lines of evidence indicate that the opening of a single L-type Ca 2 1 channel triggers a Ca 2 1 spark in an obligatory fashion (Niggli and Lederer, 1990; Cannell et al., 1995; Lopez-Lopez et al., 1995; Santana et al., 1996; Lipp and Niggli, 1996; Collier et al., 1999). This tight coupling between gating of the voltage-dependent sarcolemmal channel and the sarcoplasmic reticular release channel underlies the full mobilization of Ca 2 1 that occurs in cardiac myocytes with each action potential. RYRs are widely expressed in nonsarcomeric (smooth) muscle, neurons, and nonexcitable cells, although their role in calcium release and cellular signaling is poorly understood. In smooth muscle, RYRs are expressed on the sarcoplasmic reticulum (Carrington et al., 1995; Lesh et al., 1998) and triggered Ca 2 1 release has been inferred from measurements of calcium-activated membrane currents and spatially averaged [Ca 2 1 ] i (Zholos et al., 1992; Ganitkevich and Isenberg, 1992, 1995), but little direct evidence of CICR exists and the function of RYRs in E-C coupling is poorly understood. Spontaneous Ca 2 1 sparks have been reported in smooth muscle (Nelson et al., 1995; Mironneau et al., 1996; Gordienko et al., 1998), and recent experiments combining confocal microscopy and patch-clamp techniques have demonstrated localized calcium release during depolarizing voltage-clamp steps (Arnaudeau et al., 1997; Imaizumi et al., 1998), further supporting the existence of CICR in smooth muscle. Here we show that the L-type Ca 2 1 Address correspondence to Michael I. Kotlikoff, Department of Animal Biology, University of Pennsylvania, 3800 Spruce Street, Philadelphia, PA 19104-6046. Fax: 215-573-6810; E-mail: [email protected] ✪ The online version of this article contains supplemental material. 1 Abbreviations used in this paper: CICR, calcium-induced calcium release; E-C, excitation–contraction; RYR, ryanodine receptor; SR, sarcoplasmic reticulum. on N ovem er 4, 2016 D ow nladed fom Published May 1, 2000 /content/suppl/2000/04/24/115.5.653.DC1.html Supplemental Material can be found at: 654 Calcium-induced Calcium Release in Smooth Muscle: “Loose Coupling” current (I Ca ) evokes CICR in single urinary bladder myocytes and establish the relationship between L-type Ca 2 1 channel opening and RYR-mediated calcium release in smooth muscle. M A T E R I A L S A N D M E T H O D S